HIGH SENSITIVE RADIATION DETECTOR FOR RADIOLOGY DOSIMETRY

Transcription

1 HIGH SENSITIVE RADIATION DETECTOR FOR RADIOLOGY DOSIMETRY Mauro Valente 1, Francisco Malano 2, Wladimir Molina 3 and Jose Vedelago 4 1 Instituto de Física E. Gaviola CONICET & LIIFAMIRx (Laboratorio de Investigaciones e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X) Oficina 102 FaMAF - Univ. Nac. de Córdoba Ciudad Universitaria, 5000 Córdoba, Argentina valente@famaf.unc.edu.ar 2 Instituto de Física E. Gaviola CONICET & LIIFAMIRx (Laboratorio de Investigaciones e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X) Laboratorio 448 FaMAF - Univ. Nac. de Córdoba Ciudad Universitaria, 5000 Córdoba, Argentina malano@famaf.unc.edu.ar 3 Instituto Venezolano de Investigaciones Cientificas IVIC, Caracas; Venezuela & LIIFAMIRx Laboratorio de Investigaciones e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X Laboratorio 448 FaMAF - Univ. Nac. de Córdoba Ciudad Universitaria, 5000 Córdoba, Argentina wmolina@ivic.gob.ve 4 LIIFAMIRx (Laboratorio de Investigaciones e Instrumentación en Física Aplicada a la Medicina e Imágenes por Rayos X) Laboratorio 448 FaMAF - Univ. Nac. de Córdoba Ciudad Universitaria, 5000 Córdoba, Argentina josevedelago@gmail.com 331

2 Abstract Fricke solution has a wide range of applications as radiation detector and dosimetry. It is particularly appreciated in terms of relevant comparative advantages, like tissueequivalence when prepared in aqueous media like gel matrix, continuous mapping capability, dose rate recorded and incident direction independence as well as linear dose response. This work presents the development and characterization of a novel Fricke gel system, based on modified chemical compositions making possible its application in clinical radiology. Properties of standard Fricke gel dosimeter for high dose levels are used as starting point and suitable chemical modifications are introduced and carefully investigated in order to attain high resolution for low dose ranges, like those corresponding to radiology interventions. The developed Fricke gel radiation dosimeter system achieves the expected typical dose-dependency, actually showing linear response in the dose range from 20 up to 4000 mgy. Systematic investigations including several chemical compositions are carried out in order to obtain a good enough dosimeter response for low dose levels. A suitable composition among those studied is selected as a good candidate for low dose level radiation dosimetry consisting on a modified Fricke solution fixed to a gel matrix containing benzoic acid along with sulfuric acid, ferrous sulfate, xylenol orange and ultra-pure reactive grade water. Dosimeter samples are prepared in standard vials for its inphantom irradiation and further characterization by spectrophotometry measuring visible light transmission and absorbance before and after irradiation. Samples are irradiated by typical kv X-ray tubes and calibrated Farmer type ionization chamber is used as reference to measure dose rates inside phantoms in at vials locations. Once sensitive material composition is already optimized, dose-response curves show significant improvement regarding overall sensitivity for low dose levels. According to the obtained results, it is found that the proposed method shows satisfactory reliability and good enough performance for a promising radiation dosimetric system. Keywords: Radiation detection; Fricke gel dosimetry; Radiology 332

3 1.- INTRODUCTION Ionizing radiations are crucial to the diagnostic and treatment of diseases due to their ability to interact and to transmit through human tissues, thus motivating to increase applications of ionizing radiation based techniques worldwide. As a direct consequence of the growth in the use of ionizing radiation in medicine it is indeed the increasing problematic associated with the determination of the absorbed dose in the different clinical applications. In this sense, the mechanisms for estimating the dose in medical practices involving ionizing radiations have become critical and actually there exists a need to develop and deploy new dosimetry techniques to cover a broad range of techniques for both diagnostic and therapy medicine. The present study proposes the development of a ferrous-xylenol gel (Fricke) dosimeter with dose sensitivity characteristics allowing to perform of absorbed dose measurements for medical application in both therapy and diagnosis practices. Initializing the study with a material already established for the measurement of high radiation doses [Valente 2007] there were carried out investigations by modifying it and also incorporating new chemical components, like benzoic acid, in its preparation. Comparisons between the developed novel Fricke gel dosimetric system with a conventional dosimetry system were accomplished with the aim of its validation as a reliable dosimetry system. Irradiation experiences were realized according to typical setups of computed tomographic (CT) practices. Noteworthy that dosimetry systems based on Fricke gel have significant advantages, highlighted tissue equivalence, beam quality independence (a priori, at least to conventional techniques, ie not hadronic) and regardless of both the quasi linear energy transfer and dose rate [Valente 2007; Mariani et al., 2007; Vanossi et al., 2008; Valente et al., 2012]. Actually, it has been demonstrated ability to obtain three-dimensional dosimetry mappings, fact that all these features are executed with a moderate cost of production, once 333

4 having expertise knowledge and appropriate instrumentation [Valente 2007; Valente et al., 2007; Carrara et al., 2007]. The aim of this work was to develop, characterize and preliminary implement a novel dosimetric radiation detector with high enough sensitivity in order to be capable of performing reliable absorbed dose distribution measurements for low dose level clinical applications as required by radiological procedures [Valente et al., 2012; Molina 2013]. 2.- MATERIALS AND METHODS Materials and Instrumentation A self-built computed tomography facility provided with dedicated mechanical and electronic systems controlled by software was used to irradiate dosimeter samples positioned in sample holders. In addition to mechanics and electronics, the CT facility incorporates detection equipment incorporating a flat panel Varian model PaxScan which allows energy dynamic range of 40 kev to 150 kev. A continuous emission X-ray tube was used as a radiation source with electron current intensity within 5-50 ma, voltage acceleration potential in the range kvp and provided by different available anode materials. Adaptations to the X-ray sources, mechanics-electronics devices, radiation detection systems and dedicated post-processing software are the mainly constituents of the CT facility, which is optimized for high spatial resolution thus being a micro-ct ( CT), actually. The facility allows to perform both digital radiography and tomography imaging. A spectrophotometer 1205 Vis from UNICO company was used for measuring both absorbance and transmittance of the samples covering a spectrum within a wide dynamic range from quite infrared up to quite ultraviolet ( nm), reproducibility of (±1 nm), 334

5 a transmittance range from 0% to 125% and a absorbance photometric accuracy of (±0, 004OD). Samples of tri-distilled water in UV quartz container with tolerance of 1% corresponding to 220 nm were used as the reference standard for the measurement of absorbance difference. Fricke gel samples were prepared using standard spectrophotometric acrylic containers. An ionization chamber PTW-Freiburg TN with certified calibration factor in water N D W,Q 0 was used as conventional reference dosimetry system for calibration and comparisons. A phantom of 3 cm in diameter and 5 cm high, made of 2 mm acrylic walls and filled with water was designed for tomographic applications. Additionally, there were designed special adaptations (see Figure 1) devoted to estimating the quite homogeneous dose at center of phantom. The adapters ensure that both Fricke gel and ionization chamber remain properly positioned inside the phantom so that centers of both sensitive volumes match the same position. Figure 1.- Cylindrical phantom of 30 mm diameter, 40 mm height and 2 mm wall thickness along with special adapters (left), Fricke gel dosimeter sample accommodated in water filled cylindrical phantom (middle) and tomographic irradiation setup (right). 335

6 2.2.- Fricke gel characterization methods The development of a new dosimeter was carried out introducing modifications to the initial reference consisting on a Fricke gel dedicated to the high radiation dose levels estimation [Valente 2007; Valente et al., 2007] composed by a solution of mm of porcine skin gel 300 Bloom, 0.6 mm of ferrous ammonium sulphate (Mohr s salt), 8 mm of sulfuric acid, 2 mm of xylenol orange and 96% of tridistilled water. The chemical composition of sensitive material was modified in the first instance, using suggestions provided by bibliography [Valente 2007, Valente et al., 2012]. For the new design the procedure consisted on the variation in a controlled way of one component at a time of the solution; while keeping remaining components unchanged. In this way, although the results were obtained for the dosimeter output characteristics with respect to these variations, they proceeded to attain the most favorable outcome in view of achieving increments in both sensitivity and stable behavior at the time of measurement (linearity well defined with minor as possible deviation margin between measurements), whose chemical composition resulted from variations in the following ranges: benzoic acid between mm, sulfuric acid between mm, ferrous ammonium sulfate from mm and xylenol orange between mm. Characterization was performed by considering the relationship between the difference in absorbance of non-irradiated and irradiated samples with water reference using X-ray tube setup of 40 kvp and 40 ma. Fricke gel samples were placed inside the cylindrical phantom for its irradiation, as shown in Figure 1. Any system to be considered a suitable dosimeter has to fulfill acceptable characteristics of reproducibility and independence with the radiation beam quality [Valente 2007], which were tested to judge if the new solution is applicable in low dose level dosimetry. Reproducibility, being one the most relevant properties of the dosimeter, was studied by experiments aimed to test if the new solution could reproduce tolerably dose dependence 336

7 response, assessing in all cases as variables the minimum values of sensitivity and linearity of the dose dependence, expecting descriptive similarities between the evaluated solutions. It was also investigated dependence with respect the characteristics of the radiation beam quality. The experiments conducted to this aim were devoted to evaluate the independence of the new solution with respect to the characteristics of the radiation beam. It was fixed the X-ray tube electron current at 40 ma and radiation quality was varied by changing accelerating voltage, thus getting beams with different half value layer (HVL). Measurements were performed for 125 and 250 mgy dose values at dosimeter sample position, determined by ionization chamber. Finally, there were carried out specific experiments in order to preliminary state the overall performance of the developed system in clinical-like situations. Tomographic images were made with a setup of X-ray tube of 50 kvp and 5 ma, respectively; with both a Zirconium filtering of 0.5 mm and aluminum filtering of 1 mm at the exit window of beam. This filtering process ensured high definition images and little detector saturation. The setup of Fricke gel in the tomographic system is presented in Figure 1. Hence, using an absorbance difference of each sample (irradiated and non-irradiated) with water reference and obtained the calibration curve from the solution the absorbed dose was assessed by Fricke gel dosimeter. The calibration curve should match the linear regression that predicts the response of the dependent variable with respect to changes in the independent variable. Therefore, it had to be adjusted to regression model with the dependent variable corresponding to the variation of the optical density differences ( OD); calculated as the ratio of logarithms of transmittance before and after irradiation; of the irradiated with respect to the non-irradiated sample using as independent variable the absorbed dose. Then, operational transformations of OD provided the corresponding values of unknown absorbed dose. Finally, comparisons were performed between read outs by both dosimetry systems considering that they might be associated to homogeneous distribution meaning that lectures corresponded to quite punctual dose values representing the mean effective 337

8 absorbed dose in sensitive volume. Dose rate for one minute of irradiation was determined, then measuring the processing time of CT it was estimated the total dose during the whole imaging process. Dose rate values were measured by calibrated ionizing chamber maintaining it at phantom center and ensuring stability during the motion. 3.- RESULTS Development and preparation of Fricke gel sensitive material The first step was to study the effect of changing the acidity (ph) level of the standard (high dose level optimized) material. Solutions prepared with 2.5 mm and 10 mm of sulfuric acid showed a dramatic increase in optical density even under protection (avoiding light exposition and controlling temperature below 15 degrees), even just 5 minutes elapsed. Therefore, it evidences a high degree of chemical instability, which generated spontaneous oxidation. In fact, the solutions were oxidized after 25 minutes making them unusable for subsequent applications. The addition of 5 mm of benzoic acid demonstrated to provide stability compared to previous solutions, since five minutes after elaborated, the preparation showed no changes visible in its optical density, remaining stable up to 30 minutes of observation after processing in conditions of shelter, even without assessing the sensitivity of this composition. This empirical observation, although being the sensitivity not yet quantitatively evaluated, suggested that this concentration of benzoic acid appeared as convenient for the preparation of the Fricke gel solution. Further experiences supported this assumption [Valente et al. 2012]. With respect to the variation between 15 and 35 mm of sulfuric acid, it was found that the increase thereof in the solution resulted proportionally related to the increase in the obtained sensitivity, as shown in Figure 2. The solution prepared with 35 mm of sulfuric acid concentration was the most sensitive one but also the less stable one, 25 mm solution proved to be sensitive but also committed 338

9 in its stability. Considering the priority requirement that desired solution has to satisfy the conditions of greater sensitivity but with a good level of stability, a new study was designed to evaluate the stability of the solution according to the time elapsed in measurement conditions using two compositions with 32 mm and 25 mm of sulfuric acid, respectively. The obtained results are reported in Figure 2. The solution of 25 mm proved to be more stable compared to the corresponding to 32 mm. However, on the other hand, it was found that for elapsed time within minutes, the solution with 32 mm of sulfuric acid is almost twice as sensitive to its counterpart. In view of these results, it was analyzed the possibility that both compositions were at the compromise boundary between stability and sensitivity. Therefore, analyzing the linear trend of the sensitivity response as a function of the increasing the amount of sulfuric acid in the solution (as evidenced in Figure 2), it was suitable to continue with further variations of component concentrations setting the quantity of sulfuric acid to 29 mm, which is the representative midpoint of the behavior between the two concentrations studied. The results obtained of varying the concentration of ferrous ammonium sulfate basically showed increased sensitivity for greater amounts of ferrous ammonium sulfate available to interact and react with the incident radiation, as shown in Figure 3. Particularly, the composition corresponding to 0.3 mm of ferrous ammonium sulfate showed sufficient enough sensitivity to differentiate low dose values as 50 mgy also having good stability. Contrary, the solution with 0.2 mm of ferrous ammonium sulfate was not able to differentiate dose values lower than 175 mgy. Finally, it was found that preparation with the concentration of 0.5 mm ferrous ammonium sulfate did not exhibit linear respond within an acceptable large enough dose range. Therefore, the concentration of 0.3 mm of ferrous ammonium sulfate was used to continue the elaboration process aimed at the development of Fricke gel system with sensitivity for low levels of radiation dose. Regarding the variation of xylenol orange, the obtained results showed that this 339

10 component is closely related to the stability of the system. Stability studies regarding an elapsed time post-irradiation suggest that the composition prepared with 0.1 mm xylenol orange is more stable, as reported in Figure 3. Figure 2.- Dose-response from samples of Fricke gel dosimeter with different concentrations of sulfuric acid: 15 mm in squares, 25 mm in triangles and 35 mm in circles (top) along with output stability (bottom). 340

11 Figure 3.- Dose-response from samples of Fricke gel dosimeters with different concentrations of ferrous ammonium sulfate: 0.5 mm in circles, 0.3 mm in triangles and 0.2 mm in squares (top) and dose-response dependence on xylenol orange amount: 0.1 mm in circles and 0.3 mm in squares (bottom). The composition prepared with 0.1 mm xylenol orange was considered more stable than its counterpart, so this was the amount of xylenol orange chosen for the development towards a more stable and sensitive gel dosimeter. 341

12 Finally, the composition determined as that corresponding to the higher sensitivity to the purposes of this study consists on: 5 mm benzoic acid, 29 mm sulfuric acid, 0.3 mm ferrous ammonium sulfate and 0.1 mm xylenol orange, all components suspended in a gel matrix gel mm porcine skin gel 300 Bloom gel to 96% of volume with tri-distilled water. The output (dose-response curves) obtained through the change in optical density of the solution when exposed to radiation beam with respect to the absorbed dose showed a linear behavior with two slopes well defined, as seen in Figure 4. Moreover, it was possible to achieve good enough resolution for dose levels even lower than 40 mgy that were clearly distinguishable from background. The first (low dose level) linear trend extended up to values around 250 mgy. Then, there appeared an evident change in the slope of the linear response, whose extension arrived at least up to dose values around 3 Gy. This behavior is due to changes in the nature of the solution due to the exhaustion of any of its components. In fact, the sensitivity tests indicate that the component that mainly affects this behavior is the ferrous ammonium sulfate. Incorporating processing protocols and process improvements are achieved sufficient sensitivity to distinguish doses levels below 20 mgy maintaining linearity behavior with a double slope at a point 250 mgy and consistency similar for all preparations Characterization of the novel high sensitive Fricke gel dosimeter Three different elaboration of the optimized composition were evaluated changing absorbed dose ranges during irradiation. Sample positioning in phantom and further read out were maintained, hence ensuring that the obtained results could be further compared in order to investigate if the output remain stable regardless of the method of characterization. 342

13 Figure 4.- Dose-response of high sensitive dosimeter (benzoic Fricke gel dosimeter - BFGD) irradiated at different dose ranges: Gy in diamond, Gy in squares and Gy in triangles. The first solution was evaluated for the range of Gy measuring intermediate values at intervals of 0.01 Gy. The second was tested in the range of Gy with intermediate values at intervals of 0.05 Gy. Finally, the last solution was evaluated for a range Gy measuring intermediate values at intervals 0.5 Gy. As presented in Figure 4, it was appreciated the same behavior of linearity between solutions regardless of characterization method. Table 1 reports measurements of average absorbed doses corresponding to different beam qualities (accelerating voltages of 25 kvp and 45 kvp), thus supporting that dosimetry 343

14 system response seems to be independent of beam quality, due to statistically non distinguishable results. Table 1.- Dosimeter response ( OD) for different radiological beam qualities Beam quality [kvp] Reference delivered dose: 125 mgy Reference delivered dose: 250 mgy ± ± ± ±0.06 This evidence preliminary suggests that the novel developed sensitive dosimeter exhibits response independence respect to beam quality, as less in radiological applications Implementation of the novel dosimeter for radiology Tomography imaging of cylindrical phantom with ionization chamber inserted was performed in correspondence with Figure 5. The reconstructed images showed an internal structure of the ionization chamber clearly differentiated from the water surrounding it. Ionization chamber sensitive (ionization) volume consists mainly on air and electronics incorporates metallic pieces therefore arising discrepancies between the sensitive volume of the ionization chamber and water, conventionally considered as reference for human (soft) tissue; ie. being radiological equivalent to water. Contrary, in the case of the corresponding tomographic images obtained from Fricke gel dosimeter presented in Figure 5, it was confirmed that when placing the Fricke gel dosimeter within a water equivalent medium, there are quite non differences in tomography signal suggesting very similar radiation absorption and scattering properties between water and dosimeter. Therefore, indicating good water equivalence, thus meaning (soft) tissue equivalence for clinical applications, as mentioned. The calibration curve was obtained for the solution implemented in this test getting absorbance sensitivity of (0.65 ± 0.001) and correlation coefficient R 2 = 0.998, as shown in 344

15 Figure 6. This is evidence of a linear fit with a strong tendency deterministic, where 99.8% of the data from the optical density difference fit the regression line. Figure 5.- Coronal slice obtained from computed tomography of water equivalent phantom with ionization chamber (left) and novel Fricke gel dosimeter (right) inserted. Contrast was adjusted to better highlight details. Applying operational transformation, it was calculated the average of the measurements obtained in the estimation of dose by means of the novel Fricke gel dosimeter. Additionally, the developed system was tested using a simple and typical setup for conventional radiography. Dosimeter vials were placed inside a water-equivalent phantom exposed to 45kV X-ray beam imaged (using long acquisition times) with a bi-dimensional digital detector, as sketched in Figure

16 Figure 6.- Calibration curve of high sensitive Fricke gel dosimeters used for CT applications. Figure 7.- Sketch of irradiation configuration (upper view) including incident X-ray beam, cylindrical water phantom containing 5 dosimeter vials and 2D detector in CT set up. 346

17 The irradiation process for comparisons lasted 12 minutes using an intensity modulated beam. Hence, multiplying the recorded time considering a dose rate of 1 minute it was obtained estimates of absorbed dose to the ionization chamber, as reported in Table 2. Table 2.- Dose measurements in CT irradiation Dosimeter location Ion. Chamber [mgy] Fricke gel dosimeter [mgy] Center (1 st experiment) 134.1± ±3.1 Center (2 nd experiment) 134.0± ±2.4 Center (3 rd experiment) 134.0± ±1.9 Up (average all experiments) 78.6± ±8.2 Down (average all experiments) 100.6± ±6.4 Left (average all experiments) 123.7± ±4.5 Right (average all experiments) 100.0± ± DISCUSSION First of all, it should be emphasized that the overall performance of the developed dosimetric system shows typical characteristics and satisfies conventional requirements of gel dosimeters [Valente 2007]. As expected, the presence of linearity in dose-response is reduced to specific dose ranges. Moreover, the extension of this linearity range shows to be the strongly correlated with linear slope; which represents the sensitivity of the system. The behavior at doses lower than linearity range corresponds to threshold effects exhibiting supra linear trend; whereas for doses higher than linearity range the output presents typical saturation effects, then sub linear trend, due to extinction of available reactive components. It is actually expected that both sulfuric and benzoic acids concentrations should significantly influence the output, because the change in medium ph that highly influence overall reactive grade. Similarly, concentration of ferrous ammonium sulfate being the component providing the reactive agents (ferrous oxide) affects output. These results are in agreement with previous works [Valente 2007; Valente et al., 2012; Molina 2013]. 347

18 The selection of the optimal chemical composition for low dose level dosimetry was based on the compromise between high sensitivity (high slope values), extension of linearity range and composition output stability, both after preparation and after irradiation. Regarding preparation of Fricke gel sensitive material, the approach of testing the output response in correspondence to different chemical compositions varying one component at a time allowed to suitably improve the dose response sensitivity. Once a high enough sensitive composition was achieved, further studies about stability were conducted in order to extend the available parameters for helping in composition selection. The dedicated optical analysis technique for the characterization of the high sensitive Fricke gel dosimeter demonstrated remarkable capability for assessing absorbed dose by means of transmission/absorption measurements. The implemented methodology appearing as optimal for analyzing homogeneous dose distribution within the sample, however would need to be suitably modified if non uniform dose distribution would be necessary to measure. In fact, one of the characteristics of the analyzing method that might be considered a priori somehow as a limitation is that recorded dose values correspond to the integral along the optical path, thus providing an estimation of an effective dose value that obviously is not able to distinguish different doses along the integration path. So, that is the reason because Fricke gel samples should be prepared in small enough containers so that when irradiated it might be reasonable to consider that absorbed dose is quite constant within sample path. Nevertheless, this potential drawback is suitably overcome by an improved 2D analyzing technique based on almost monochromatic sample transmission imaging [Valente 2007]. The obtained results for measuring low level absorbed dose in CTlike set up, some of them summarized in Table 2, evidence that the developed novel dosimetry system is capable to assess similar dose estimations than ionization chamber here employed as reference conventional system. Although the noticeable overall agreement between high sensitive Fricke gel dosimeter and conventional method, it has to be remarked, however, that ionization chamber and Fricke gel dosimeter measurement volumes do not agree, therefore it should be expected that different dose read out values would be recorded if non uniform irradiation is performed. 348

19 Moreover, as pointed out by results reported in Figure 5, the presence of non-water equivalent components of the ionization chamber might affect, even in minor but potentially non negligible way, the radiation field, thus arising non desired measurements alterations. 5.- CONCLUSIONS As a general comment, it has to be emphasized that it was successfully accomplished the purpose of developing, characterizing and preliminary implementing a novel tissueequivalent high sensitive Fricke gel dosimeter. According to the obtained results, it was demonstrated that novel Fricke gel dosimetry system designed for low levels dose certainly exhibits linear response and actually achieves high sensitivity. Strictly, it can be stated that the developed system is capable of reliably measuring dose values within the range of mgy recording read outs being well distinguishable from the background. In addition, the developed dosimetry system shows similarity in the dose record to conventional system, being indeed an indication that the developed system might constitute a promising tool as radiation detector with dosimetric capability. Therefore, the obtained results suggest that the developed system can be considered potentially applicable to dosimetry in radiology and radiotherapy practices. Acknowledgments This work is part of the project Innovation Systems Research for Radiation Dosimetry: phase I (ISIDORA I) under the direction of Dr. Mauro Valente and funded by the Secretary of Science and Technology at National University of Cordoba, Argentina in conjunction with the Ministry of Science and Technology of the Argentine Republic. 349

20 REFERENCES Carrara M; Gambarini G; Tomatis S; Valente M. (2007). Dose distribution measurements by means of gel-layer dosimeters. evaluation of algorithms for artefacts amendment. Nucl. Instr. Rad. Res. A 579: Mariani M; Vanossi E; Gambarini G; Carrara M; Valente M. (2007). Preliminary results from polymer gel dosimeter for absorbed dose imaging in radiotherapy. Radiation Phys. Chem. 76(8): Molina W. (2013). Desarrollo y caracterización de un sistema de Fricke gel para dosimetria en radiodiagnóstico. Tesis de maestría del Instituto Venezolano de Investigaciones Científicas. Venezuela. Valente M. (2007). Fricke gel dosimetry for 3D imaging of absorbed dose in radiotherapy. PhD Dissertation University of Milan. Italy. Valente M; Gambarini G; Bartesaghi; Gambarini G; Brusa D; Castellano G; Carrara M. (2007). Fricke gel Dosimeter Tissue Equivalence: a Monte Carlo Study. Proccedings of the 10 th International Congress on Astroparticles, Particles and Space Physics, Detectors and Medical Physics and Applications. Villa Olmo, Italy: Valente M; Molina W; Tirao G; Graña D; Perez P. (2012). Novel tissue-equivalent chemical dosimeter optimized for radiological low dose levels. Proceedings of the 11 th International Low Radiation Conference. Lyon, France. Vanossi E; Gambarini G; Carrara M; Valente M. (2008). Study of polymer gel for dose imaging in radiotherapy. Radiation Meas. 43(2):